The following section can be used as reference for all of the custom brush tool options available when making a pencil, pen, or paint brush.
The Brush size menu options in the Sub tool detail dialog box.
Change the initial size of the tool. The greater the number of the brush size, the larger the brush. You can always change the size while you're working with the brush, but this will set the initial size.
This keeps the brush tip the same size in proportion to the zoom of your working canvas. Turn this on if you zoom in and out of your canvas a lot and don't want to change your brush size all the time. While you're zoomed in closer to the canvas, lines will appear ...